1. Field of the invention
This invention, relating to fabrication methods and equipment for granulated powders formed from rare earth containing alloys such as R--Fe--B-type and R--Co-type alloys, regards the production of isotropic granulated powders by stirring a slurry of the said rare earth containing alloy powders, spraying within the chamber of a spray dryer apparatus to form liquid droplets and instantaneously dry solidifying them, and the production of anisotropic granulated powders by applying a magnetic field to the slurry to orientate the said powder particles, spraying in the said chamber to form orientated liquid droplets and instantaneously dry solidifying them. The invention describes these fabrication methods and the fabrication equipment for the production of isotropic and anisotropic granulated powders with good magnetic properties where the flow and lubrication properties of the powders at the time of compression molding are improved, and the molding cycle and dimensional precision are also improved.
2. Description of the prior art
These days, with the production of small, light-weight motors and actuators used in everything from home electrical goods to computer peripherals and motor vehicles, much effort is being made to increase their efficiency. This means small, light-weight and thin magnetic materials to be used in these motors are also being sought.
Currently, typical sintered permanent magnetic materials are ferrite magnets, R--Co-type magnets and R--Fe--B-type magnets previously suggested by the applicants (Japanese Patent Publication SHO 61-34242).
Of the above, rare earth magnets in particular, such as R--Co-type and R--Fe--B-type magnets, have exceptional magnetic characteristics compared to other magnetic materials.
The above rare earth magnets, for example, the R--Fe--B type sintered permanent magnets have extremely good magnetic properties and have a large energy product ((BH)max) that exceeds 40 MGOe, with being over 50 MGOe as the greatest energy product. In order to achieve this, it is necessary to grind alloys of the required composition to powders with an average particle size of 1.about.10 .mu.m.
However, as the particle size of the alloy powders is made smaller, the flowability of the powders when molding becomes worse, and together with a reduction in the dispersion of the molded product density and the molding apparatus life span, there exists a dispersion in the dimensional precision after sintering which causes difficulties particularly in the fabrication of thin films and small shapes.
Further, rare earth magnets contain rare earth elements and iron which are easily oxidized in the atmosphere, and such, as the alloy powder particle size is made smaller, degradation of the magnetic properties due to oxidation becomes a problem.
In order to improve the molding characteristics, additives to the alloy powders before molding have been suggested such as polyoxyethylene alkyl either (Japanese Patent Publication HEI 4-80961 (JPB4- 80961)), or further adding of paraffin or stearate (Japanese Patent Publication HEI 4-80962, Japanese Patent Publication HEI 5-53842), or oleic acid (Japanese Patent Publication SHO 62-36365).
However, although the molding characteristics can be improved somewhat, the beneficial effects are limited, and the problems in molding thin films or small shapes are as yet unsolved.
Further, as well as adding binder and lubricants to the above, other methods have been proposed to improve the molding characteristics in the production of thin films and small shapes. These include granulating and molding after adding and mixing a lubricant consisting of myristic acid ethyl and oleic acid, saturated fatty carboxylic acids and unsaturated fatty carboxylic acids to the powders before molding (Japanese Patent Laid Open SHO 62-245604, JPA62- 245604), or molding after adding saturated fatty carboxylic acids and unsaturated fatty carboxylic acids to a paraffin mixture, after mixing and granulating (Japanese Patent Laid Open SHO-63-237402).
However, in the above methods the binding force between the powder particles is insufficient, and as the granulated powders break apart easily, obtaining a sufficient particle flow is a problem.
In order to improve the molding characteristics and increase the binding force between powder particles, one method might be to increase the amount of added binders and lubricants. However, the amount that can be added is limited due to the fact that, as the amount of additives is increased, a reaction occurs between the R component in the rare earth containing alloy powders and the binder causing an increase in the residual oxygen and carbon content in the sintered material leading to a degradation in the magnetic properties.
Although not directly related to rare earth containing magnetic alloy powders, binders for compression molding of Co-type superalloys have been proposed where, for that particular alloy powder, a composition of mixed glycerol and boron was used containing 1.5.about.3.5 wt % methyl cellulose and other fixed amounts of additives (U.S. Pat. No. 4,118,480). As well, binders for injection molding of alloy powders for tools, consisting of a particular composition, have been proposed where for that particular alloy powder, a composition was used where plasticizers, such as glycerol and water, lubricants, such as wax emulsion, and parting agents were added to 0.5.about.2.5 wt % methyl cellulose (Japanese Patent Laid Open SHO 62-37302).
However, in order to maintain fixed flow and mold strength characteristics, for each particular alloy powder, because, as in the above examples, more than 0.5 wt % is using comparatively a lot of binder, it is essential to add various binder additives, for example, adding equal amounts of plasticizers such as glycerol to methyl cellulose, and as such, even after injection or compression molding, degreasing and sintering, there still remains much residual carbon and oxygen, and particularly in the case of rare earth magnets, the degradation in the magnetic properties makes these methods unsuitable.
For ferrite oxide powders, methods such as adding 0.6.about.1.0 wt % polyvinyl alcohol as a binder to powders of an average size of less than 1 .mu.m, then producing granulated powders using a spray dryer apparatus and molding and sintering the said powders, are known.
However, for these oxide powders, as more than 0.6 wt % is using a large amount of binder, even after the degreasing process has been carried out there remains much carbon and oxygen in the sintered product and as such these are very easily oxidized or carbonized. So, as the degradation in the magnetic properties dale to even a small amount of oxidation or carbonization is extreme for the rare earth containing alloy powders of this invention, the above methods used for oxides cannot be simply applied here.
In particular, in the case of oxides, even if one uses a comparatively large amount of binder, as degreasing and sintering can be done in air, one can control to some extent the amount of residual carbon by combusting the binder when degreasing and sintering. However, for the rare earth containing alloy powders of this invention, as the magnetic properties are degraded by oxidation it is not possible to perform degreasing and sintering in air and so adding large amounts of binder has an enormously bad influence on the magnetic properties of the sintered magnet obtained.
Therefore, although various methods have been proposed to improve the molding characteristics by adding various binders and lubricants to alloy powders before molding and then granulating them, in each case, these present problems for the fabrication of rare earth magnets having good magnetic properties into thin film or small shape forms, as has been required in recent years.